The present invention relates to fuel burners and particularly relates to a low emissions fuel burner e.g. utilized for industrial drying processes.
High capacity fuel burners are generally used in industries requiring drying of various materials. For example, such burners are required for operating large rotary aggregate dryers and for kiln drying and processing of lime, sand, bauxite, coal, cement and the like.
In drying aggregate for use in asphalt roads, for example, a fuel burner of this type is employed in conjunction with a rotating drum. Wet aggregate is introduced into one end of the drum and veiled as the drum rotates such that the hot gases emanating from the fuel burner pass through the falling aggregate within the drum, removing the moisture from the aggregate. In a typical installation, the exhaust gases are passed through a baghouse which removes particulates and exhausts the gases to the atmosphere. Environmental considerations, however, require a low pollution emissions burner, particularly a burner providing low emissions of nitrous oxides (NOx). With large-scale burners of this type, the problem of providing low emissions, particularly nitrous oxides, is ongoing. Accordingly, there is a need for a high capacity, low emissions burner for use in industrial processes as described.
In a preferred embodiment of the present invention, there is provided a high capacity, low pollution emissions burner which particularly affords low emissions of nitrous oxides. To accomplish the foregoing, the preferred embodiment of the present invention provides a combination premix and diffusion burner. Particularly, the high capacity burner hereof is provided with a diffusion burner head along the central axis of the burner and which diffusion burner provides high flame stability. While diffusion-type burners typically have substantial NOx emissions, the present invention combines a diffusion burner and a premix burner such that the diffusion burner operates at reduced capacity and its flame serves primarily to stabilize the premix burner flame during main or high firing. Thus, the diffusion burner affords burner stability throughout the entire operating range of the overall burner. It also operates at a constant fuel rate with maximum swirl air throughput within the capacity of the burner""s high pressure fan to cool core portions of the diffusion flame which produce NOx. By lowering the core temperature of the diffusion flame, the NOx emissions resulting from the diffusion flame are reduced.
The heat output of the burner is advantageously supplied principally by the premix multiple burners. By arranging the premix burners in an array about the central axis of the diffusion burner, the major heat source, for example, for drying aggregate, is displaced away from the centerline of the burner and provides improved aggregate drying. Also, it will be appreciated that premix burners typically have a narrowed stability range in comparison with diffusion burners. Thus, by employing a diffusion burner flame surrounded by multiple premix burner flames, the premix burner flames being stabilized by the diffusion burner flame.
More particularly, the diffusion burner has a burner head including an annular casing or venturi having openings for admitting gaseous fuel into the casing and swirl blades for swirling high pressure air supplied through the casing from a turbofan. The diffusion burner head is surrounded by an array, preferably an annular array, of premix burner heads e.g. sleeves or tubes. Each of the sleeves has a fuel supply conduit and an air supply conduit for receiving high pressure air from the turbofan. Both conduits terminate in outlet ports short of the downstream end of the premix burner sleeve. By angling the exit port of the air supply conduit into the flow of gaseous fuel discharged from the fuel supply conduit, the air and fuel gas are premixed within each premix burner sleeve. Ignition of the premix burner flame occurs generally at the downstream end of the premix burner sleeve. Pressurized air is supplied to the premix air supply conduits from the turbofan via a manifold. Secondary air is provided to the open rearward ends of the premix burner sleeves by a secondary air inlet having an adjustable damper.
In operation, after the diffusion burner is lit, maximum high pressure air is provided within the casing of the diffusion burner to provide maximum swirl energy and afford a cooling of the core of the diffusion burner flame to reduce NOx production. Notwithstanding this maximum high pressure air, the diffusion flame remains stable and anchored. Once the premix burners are lit by the diffusion flame, stability is provided the premix burner flame by the diffusion burner flame. Burner heat output is controlled by adjusting the secondary air damper supplying low pressure air to the premix burner sleeves and by modulating the fuel supply to the premix burner sleeves. The flow rate of gaseous fuel supplied to the diffusion burner is maintained constant. The fuel gas is also supplied to the diffusion burner head at a reduced rate by using smaller fuel gas admission openings in the annular casing than conventional and which, in conjunction with supplying maximum pressured air during high fire, cools the core temperature of the diffusion flame and reduces NOx production. Consequently, the overall burner has a high turndown ratio e.g. about 10:1.
In addition, water injection may be optionally provided both the diffusion and premix burners. For example, a water injection nozzle may be provided along the axis of the diffusion burner head to supply a limited quantity of water to the core of the diffusion burner flame. This water injection further cools the flame (in addition to the cooling afforded by maximizing the high pressure air to the diffusion burner) along its high temperature core where a disproportionate quantity of thermal NOx is produced. Additionally, water injection nozzles are provided about the diffusion burner head between selected premix burner sleeves to cool the premix flame during high fire operation and thereby further reduce NOx production. Also, an oil nozzle may be provided along the axis of the diffusion burner in lieu of the water injection nozzle for the diffusion burner head. The burner can then be operated solely in a diffusion mode using oil as the fuel or solely in a premix mode using only the array of premix burners and the gaseous fueled portion of the diffusion burner head surrounding the central oil nozzle.
In a preferred embodiment of the present invention, there is provided a low emissions burner comprising a diffusion burner including a casing for receiving air under pressure and having an axis, a swirler for mixing and imparting rotational motion to the air supplied through the casing and a fuel inlet to the casing for providing a stabilized flame downstream of the swirler, a plurality of discrete premix burners surrounding the air supply casing about the axis; each premix burner including a burner sleeve, a fuel supply conduit for supplying fuel into the burner sleeve and an air supply conduit for supplying air under pressure into the burner sleeve, the conduits terminating in outlet ports short of a downstream open end of each burner sleeve enabling premixing of the air and fuel supplied to the burner sleeve via the conduits and providing a substantially premix annular flame downstream of the burner sleeves surrounding and stabilized by the stabilized flame of the diffusion burner.
In a further preferred embodiment hereof, there is provided a low emissions burner comprising a diffusion burner including a casing for receiving air under pressure and having an axis, a swirler for mixing and imparting rotational motion to the air supplied through the casing and a fuel inlet to the casing for providing a stabilized flame downstream of the swirler, a plurality of discrete premix burners surrounding the air supply casing about the axis, each premix burner including a chamber, a fuel supply conduit for supplying fuel into the chamber and an air supply conduit for supplying air under pressure into the chamber, the conduits terminating in outlet ports enabling premixing of the air and fuel supplied to the chamber via the conduits and providing a substantially premix annular flame downstream of the premix burners surrounding the stabilized flame of the diffusion burner; and a water injection nozzle for the diffusion burner for injecting water into the stabilized flame of the diffusion burner to cool the core of the diffusion flame and reduce NOx production.
In a still further preferred embodiment hereof, there is also provided, in a low emissions burner having a central diffusion burner including a casing for receiving high pressure air, an inlet for supplying fuel to the casing and swirl blades for swirling the air and fuel and an array of premix burners surrounding the diffusion burner each including a burner sleeve, a fuel conduit for supplying fuel to the burner sleeve and a high pressure air conduit for supplying high pressure air into the burner sleeve for premixing with the fuel, a method of operating the burner comprising the steps of maintaining a stabilized diffusion flame by maximizing the high pressure air supplied to the casing and maintaining a constant fuel flow rate to the diffusion burner, stabilizing the premix flame using the diffuser flame; and modulating the flow of fuel to the premix burners while maintaining constant the flow of fuel to the diffusion burner.